For example, when measuring the wavelength of light it is appropriate to express the result in nanometres (nm) because we are dealing with a very small distance.

But it would be ridiculous to measure, say, the distance from the sun to the Alpha Centauri system in nanometres -- we would end up with a very large number. So, we choose an appropriate unit; in the case of this example, the light-year.

The table below gives a list of units of length (or distance) used in astronomy. Note that, although the angstrom (Å) is still widely used by astronomers as a unit for the wavelength of light, it is not a recognised unit of the International System of Units (abbreviated SI from the French Le Système International d'Unités).